In data storage devices such as disc drives, data is read from a disc by a read operation that involves a mechanical scanning motion of a disc moving relative to a read head. The read data was originally in synchronization with the mechanical motion during writing, but this original synchronization is not available at a later time during readback. The read back data needs to be resynchronized with a readback clock as the data is read and processed by a computer. This process of resynchronization is accomplished by a timing recovery circuit. The timing recovery circuit includes an oscillator that is repeatedly synchronized by the time position of transitions of the read data.
Known data storage devices use DC free coding (such as run length limited (RLL) coding) to set limits on the length of time between transitions which ensures that the oscillator does not drift too far between transitions and to limit the bandwidth of the read back signal to a narrow frequency band. Run length limited or DC free coding, however, adds undesired overhead such as slow coding rates and time delays due to decoding the DC free coding.
With the use of perpendicular recording, readback signals can be obtained that include DC levels of bits as well as transitions between bits. The need for DC free coding is considerably reduced or eliminated, however, there is a need for oscillator synchronization with perpendicular recording.
A method and apparatus are needed to avoid the high overhead of existing DC free coding schemes while providing the timing recovery needed to keep a read oscillator synchronized. Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.